I mean, considering he says that Jevil was corrupted by someone-probably the "Knight" that the Spade king was talking about-,it's all but a certainty.
I thought he was implying Jevil met Gaster
It's entirely possible Gaster is the Knight.
Maybe though right now I am thinking the Knight is basically another "player" like Chara. Kris basically wears a knightly sort of attire.
Alternatively in a twist it turns out to be your creation (the one that was discarded at the very start)
Pretty sure it's Chara. One of the first times if not the first time that the Knight is mentioned, "strange knight" appears in red text. In Undertale, red text is very strongly associated with Chara, especially in his/her Genocide dialogue. ---